Titration Part I: Qualitative Analysis
A titration reaction is a quantitative study of an acid-base reaction or a neutralization reaction. One solution of known concentration (either the acid or base) is used to determine the concentration of the other solution through a carefully monitored reaction. Typically, the titration reaction is monitored either using a pH probe or meter (which you will use today) or using a colored acid-base indicator.
Acid-base neutralization reactions can generally be described as an acid reacting with a base to yield a salt and water as shown below:
Most commonly, the H+ from the acid reacts with the OH- from the base to produce the water and the remaining ions (anion or A- from the acid and the cation or M+ from the base) combine to form the salt. (Note that in acid-base reactions, the focus tends to be on the Brønsted-Lowry definition of acids and bases.)
One of the simplest ways to classify acids is based on strength. Acids can be classified as either strong or weak. An acid (HA) that is considered a strong acid, will dissociate 100% in water, meaning there will only be H+ (or H3O+) and A- in solution and none of the original form of the acid (HA).
Any acid that dissociates less than 100% in water is considered a weak acid. Meaning if you were to look on the atomic level at a weak acid solution, you would see some H+ (or H3O+) and A-, but mostly HA.
The relative strength of acids can be found by comparing their Ka values where Ka is called the acid dissociation constant and is a measure of how much an acid dissociates in water. The more an acid dissociates in water, the stronger the acid, and the larger the Ka value.
Monoprotic or Polyprotic Acids:
Another classification of acids involves the number of H+ ions an acid can donate. A monoprotic acid will dissociate and donate one mole of H+ for every one mole of acid molecules (e.g., HCl or HNO2). A diprotic acid will produce two moles of H+ (e.g. H2SO4), a triprotic acid three moles of H+ (e.g. H3PO4), etc.
An acid-base titration curve (a plot of pH of the acid (or base) vs. volume of base (or acid) added to the solution), can give us some information about the relative strength of the acid and whether or not the acid is monoprotic or polyprotic.
In today's experiment, you will be slowly adding base to an acidic solution and monitoring the pH of the solution as the base is added. Therefore, you will most likely see a titration curve like the following:
Key features of the titration curve:
Shown below is a sample titration curve for a diprotic acid. Note the two equivalence points and two pKa values.
to Skills Section
|Back to 125 Home Page|